Traditional methods of outside diameter grinding and lapping employ a variety of machinery, each of which have certain inborn limitations relative to productivity. For example, although there are an infinite number of tangential contact points about the circumference of an abrading wheel, conventional outside diameter grinding and lapping machinery utilize only one of these points in the abrading process. Centerless through feed grinding and other types of grinding improve on this somewhat by extending one such tangential point in a generally axial direction so that several workpieces may be ground simultaneously by feeding the workpieces single file in an axial direction. Nevertheless, many other tangential points around the circumference of the abrading wheel could be utilized to effect increased productivity.
One object of this invention is therefore to provide a means of increased productivity in grinding or lapping outside diameters by simultaneous operation of the abrading cycles of a plurality of workpieces from a plurality of such points on the abrading wheel.
Another factor that has for years limited productivity in this field is the maximum attainable speed of an abrading wheel. Those who are skilled in the art generally refer to the abrading wheel speed in terms of "surface feet per minute." (SFPM) For a given abrading wheel RPM, as the abrading wheel is reduced in diameter, the surface feet per minute is reduced so that the abrasive efficiency is also reduced.
Notably, abrading wheel speeds have been limited due to certain restrictions on the RPM that are brought about by centrifugal force. At high velocities, the effect of certrifugal force may actually exceed the bond strength of the abrading wheel. The result is an outward explosion of abrading wheel fragments. As a result, a speed barrier currently exists in high speed grinding or lapping. Depending upon the type of abrading wheel which is used, the barrier generally occurs at surface speeds above about 24,00 SFPM. The actual safe operational speeds for high speed grinding wheels, however, are considerably less than the above stated 24,000 SFPM since the American Standard Safety Code Publication (ANSI-B 7.1-1970) requires that such abrading wheels be tested at the manufacturer's plant at 50% overspeed. Thus, the above stated 24,000 SFPM abrading wheel would be permitted to operate at only 16,000 SFPM.
In addition to the theoretical limit described above, in shipment, or in shop floor handling , the abrading wheel may develop undetectable hair-line cracks from rough transport. The result is again an outward explosion of abrading wheel fragments when the abrading wheel is put to use, even at SFPM otherwise considered safe. Manufacturers of high speed abrading machinery are therefore increasingly aware of the need for shatter-proof enclosures which guard against the possibility of such explosions, whether due to centrifugal force or cracks. The logical solution of first impression to this problem appears to be simply increasing the bond strength of the adhesive holding the abrading particles together. However, abrading wheels having a high bond strength are not necessarily the answer since they too have a limitation. The limiting factor, in this instance, occurs when the abrasive of the abrading wheel has become dull due to the excessive length of time which an overly strong abrading wheel bond may hold the enbonded abrasive. A variety of workpiece quality problems will then follow, as well as substantial reductions in the abrasive efficiency of the abrading wheel.
Another object of this invention is therefore to provide a means of attaining effective surface feet per minute speeds between the abrading wheel and workpiece surfaces that are greater than that attainable by rotation of the abrading wheel alone. In attaining surface speeds greater than those of the conventional art, it may be seen that a further object of the invention is to attain greater surface speeds in a safer manner.
Another limitation inherent to traditional grinding and lapping is the repeatability of sizing accuracy of the workpieces. Difficulty arises when the abrading wheel is reduced in diameter by either wear or by a resurfacing process that is commonly referred to as truing. In order to maintain contact, it is then necessary to move either the workpiece to the abrading wheel, or vice versa. This repositioning in a radial direction, is referred to as "abrading wheel compensation."
Extreme accuracy is required when compensating either the abrading wheel or the workpiece. In either instance, however, a heavily weighted housing which rests on a slide, normally supports the compensated element to act against vibration. The weight of the compensated element combined with a gathering of spent abrasive on worn machine components, may resist movement from intermittent mechanical stimuli. When compensation finally does occur, the machine may have a tendency to overcompensate beyond the required minute tolerance. Subsequent sorting operations are then necessary to segregate out-of-tolerance workpieces.
In the instance where the abrading wheel diameter has been reduced by the aforementioned truing process, accurate compensation of the abrading wheel resurfacing tool is also required. Those who are skilled in the art commonly refer to this latter process as "diamond compensation" since the resurfacing tool generally comprises substantial quantities of industrial quality diamonds. Notably, this latter compensation also occurs in a radial direction relative to the abrading wheel when performed in conventional grinding or lapping.
It is therefore a further object of this invention to provide a means of automatic abrading wheel compensation which is not affected by a gathering of spent abrasive on a mechanical slide or by intermittent mechanical stimuli from a radial direction. Since abrasive debris is commonly a problem with such machinery, it is a further object of the invention to reduce wear from abrasion by improving the design of certain other working mechanical members.